1. Field of Invention
This invention relates in general to a method for determining a vehicle""s steering ratio and more particularly for generating a map of vehicle""s steering ratio for determining the steering angle while the vehicle is turning.
2. Prior Art
U.S. Pat. No. 5,790,966 entitled xe2x80x9cMethod For Determining Steering Position of Automotive Steering Mechanismxe2x80x9d issued on Aug. 4, 1998 to Madau et al. and assigned to a common assignee relates to the determination of a steering wheel angel of a vehicle. In particular the patent teaches a method for determining a center position for the steering sensor output every time the vehicle is started.
The method employs the use of a yaw rate sensor for computing an estimate of the actual steering position to provide immediate and accurate steering position data using the steering sensor signal for controlling an automotive device. The method employs an algorithm having three levels to determine the center position.
The first level is an initial center algorithm; the second level is an interim center algorithm and the third level is final center algorithm. The first level in this prior art patent provides a very rapid estimate of the center position when the operator first starts the vehicle. Soon the computing system in the vehicle determines when it senses the steering mechanism is substantially positioned in the estimated center position. Once this switch to the second level of the algorithm is made, the computer does not return to the algorithm until the vehicle ignition is turned off and then turned on.
This invention is an improvement over the prior art in the determination of the vehicle""s dynamic steering ratio as a function of the vehicle velocity and yaw rate while the vehicle is in a dynamic maneuver.
It is a principal advantage of the present system to make an accurate determination of the center position of the vehicle while the vehicle is going at a very slow speed.
It is another advantage to map the steering ratio of the vehicle and use this map to determine the vehicle""s steering wheel angle while the vehicle is in a dynamic maneuver.
A method for mapping a vehicle steering ratio as a function of vehicle velocity and yaw rate beginning with determining the length, L, of the wheelbase of the vehicle. The understeer coefficient, Kus, of the vehicle is then determined from the mechanical design of the vehicle. The vehicle is then driven at an accelerated speed, U, from zero to a predetermined value. The vehicle is put through a turning maneuver by turning the vehicle steering wheel through a plurality of steering wheel angles at predetermined angular rate about a known vehicle center axis in the clockwise direction and repeat in the counterclockwise direction. This is typically about 1000-degree steering range.
The yaw rate, r, of the vehicle is measured at each one of the plurality of predetermined steering wheel angles during the step of turning. From these measurements, the front tire steer angles, xcex4({f}, of the vehicle is calculated according to the equation
xcex4{f}=r/U*[L+U2(Kus/g)]
for each value of U and each predetermined steering wheel angle. Next the steering ratio of the front tires is calculated from the predetermined steering wheel angle and the front tire steer angle. All of these steps are repeated for a plurality of vehicle speeds and the predetermined steering wheel angles from steering wheel lock to lock position. These results are plotted for each speed point and predetermined steering wheel angle creating a family of curves. A linear curve is calculated through each curve representing a steering ratio wherein the linear curve follows the general equation
y=mx+b 
wherein b is the value of steering ratio at zero velocity for each predetermined steering wheel angle and m is the slope of the line or curve. Points on all of the linear curves are placed as a map in a computing look-up table located in the vehicle ECU.
These and other advantages of the present invention will be found in the following detailed description and drawings.